GRAVITY-ANOMALIES, QUATERNARY VENTS, AND QUATERNARY FAULTS IN THE SOUTHERN CASCADE RANGE, OREGON AND CALIFORNIA - IMPLICATIONS FOR ARC AND BACKARC EVOLUTION

Isostatic residual gravity anomalies in the southern Cascade Range of northern California and southern Oregon are spatially correlated with broad zones of Quaternary magmatism as reflected by the total volume o f Quaternary volcanic products, the distribution of Quaternary vents, and the anomalously low teleseismic P wave velocities in the upper 30 km of crust. The orientation of Quaternary faults also appears to be r elated to gravity anomalies and volcanism in this area, trending gener ally north-south within the magmatic regions and nortwest-southeast as they enter the neighboring amagmatic zones to the north and south. Th e relationship between gravity anomalies, vent density, and fault orie ntations may indicate in a broad sense the strength of the middle and upper crust. The southern Cascade Range occupies a transition zone whe re horizontal stress is transferred from the northwest-southeast dextr al shear of the Walker Lane belt to the east-west extension characteri stic of the Cascade are in central Oregon. Faulting along north-south strikes in the volcanically active areas indicates the east-west exten sional stresses in thermally weakened crust, whereas northwest faultin g between the volcanically active areas reflects the northwest trendin g, right lateral shear strain of the Walker Lane belt. The segmentatio n of the are reflected in Quaternary magmatism may be caused by differ ential extension behind crustal blocks of the forearc rotating clockwi se with respect to North America. In this view the volcanic centers at Mount Shasta, Medicine Lake volcano, and Lassen Peak in northern Cali fornia are situated along the southern parts of the trailing edges of two distinct segments of the forearc where additional extension is imp lied by their differential clockwise rotation.